1999 SAAB 9-5 oil light

I have a 1999 saab, 9.5, 2liter turbo. The heater bypass valve was replaced 4 months ago (the start of everything). Then, about 2 months later, the turbo has gone. The garage replaced the turbo, dropped the oil pan, cleaned the pick-up tube and screen (which had a lot of sludge). Then when run, the oil light would come on after the engine warmed up. The oil sending unit was then replaced, but typically after about 20 miles of driving and coming to a stop, the light comes on. When you increase the rpm by about 2-3 hundred, the light goes out. It runs very well, sounds good. There is a little blue smoke, but the mecvhanic says that is left over from the turbo failing and oil getting into the exhaust